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Use atomic counters for ktrace buffer indexes

ktrace_enter() is consuming vast amounts of CPU time
due to the use of a single global lock for protecting
buffer index increments. Change it to use per-buffer
atomic counters - this reduces ktrace_enter() overhead
during a trace intensive test on a 4p machine from 58%
of all CPU time to 12% and halves test runtime.
Merge of xfs-linux-melb:xfs-kern:30537a by kenmcd.

  Use per-ktrace buffer atomic index counters and remove
  the global ktrace index lock that is causing performance
  problems on ktrace kernels.

/*
 * Copyright (c) 2000-2003,2005 Silicon Graphics, Inc.
 * All Rights Reserved.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it would be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write the Free Software Foundation,
 * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
#ifndef __XFS_SUPPORT_KTRACE_H__
#define __XFS_SUPPORT_KTRACE_H__

/*
 * Trace buffer entry structure.
 */
typedef struct ktrace_entry {
	void	*val[16];
} ktrace_entry_t;

/*
 * Trace buffer header structure.
 */
typedef struct ktrace {
	int		kt_nentries;	/* number of entries in trace buf */
	atomic_t	kt_index;	/* current index in entries */
	int		kt_rollover;
	ktrace_entry_t	*kt_entries;	/* buffer of entries */
} ktrace_t;

/*
 * Trace buffer snapshot structure.
 */
typedef struct ktrace_snap {
	int		ks_start;	/* kt_index at time of snap */
	int		ks_index;	/* current index */
} ktrace_snap_t;


#ifdef CONFIG_XFS_TRACE

extern void ktrace_init(int zentries);
extern void ktrace_uninit(void);

extern ktrace_t *ktrace_alloc(int, unsigned int __nocast);
extern void ktrace_free(ktrace_t *);

extern void ktrace_enter(
	ktrace_t	*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*);

extern ktrace_entry_t   *ktrace_first(ktrace_t *, ktrace_snap_t *);
extern int              ktrace_nentries(ktrace_t *);
extern ktrace_entry_t   *ktrace_next(ktrace_t *, ktrace_snap_t *);
extern ktrace_entry_t   *ktrace_skip(ktrace_t *, int, ktrace_snap_t *);

#else
#define ktrace_init(x)	do { } while (0)
#define ktrace_uninit()	do { } while (0)
#endif	/* CONFIG_XFS_TRACE */

#endif	/* __XFS_SUPPORT_KTRACE_H__ */